The detection and identification of objects at relatively close ranges requires either high range resolution or some other form of "contrast" which will allow the discrimination between targets and clutter to be made. This contrast must be sufficiently unique to allow "automatic" recognition of the contrast and thus "detection" of the desired targets (mines). Clutter, on the other hand, is all of the other fluctuations of the signal which may have some characteristics which appear as a target of interest but which are not. It is the separation of "targets" from "clutter" which limits the performance of all electronic detection systems.
In the classical "balanced coil" techniques used for metal detection (like those found in airports), a fixed frequency oscillator is used as the transmitter. The transmitter develops an "electro-magnetic field" in some form of a coil. A similar coil arrangement is used at the receiver. The receiver is designed so that the direct path energy is canceled in the receiver. The receiver output is sensitive only to disturbances in the "electro magnetic field" near the coil. (Frequently these changes are just capacitance or inductance change effects on a tuned circuit.)
Metal targets are detected as an "unbalanced" condition in the receiver. The closer the field coils are to a metal object the greater the unbalance condition. The coils are moved around over the suspected target area. The metal object is located at the point where the maximum receiver output is achieved, directly over the mine or other metal object.
These "classical" balanced coil systems work quite well when the sensor is directly over the metal object at close range. However, they lose performance rapidly as the distance between the sensor and target increases. These systems also suffer from high levels of attenuation in the salt water because of the carrier frequencies used by the oscillators.
Conventional "hand held" metal mine detection devices have provided a degree of effectiveness for the detection of shallow metal mines on land. However, such devices are typically limited to a few feet range, directly above the metal object being detected. They have been of little or no value in salt water.
Sonar systems have also been tried for mine detection in shallow salt water. However, sonar has a very difficult time operating in shallow water and near the surf. Sonar also has a difficult time with mine size targets on the bottom or which are buried.
Impulse radars (Ultra Wide Band) have been successfully used for detection of both metal and non-metal buried mines under some conditions. These 1 to 10 nanosecond impulse radars work quite well in dry soil and even in fresh water or snow. However, with salt water the attenuation of greater than 60 db per foot is a prohibitive signal loss factor.
Devices known as magnetometers have been very successful in the detection of slight variations in the earth's static magnetic field. These devices are used for submarine detection at depths of more than 500 feet.
The Navy has long used VLF communications systems which have negligible salt water signal attenuations at 10 KHz to 40 KHz carrier frequencies for communications with submarines below the sea and around the world.